注意-我们可能知道，为了使用自动大标题展开/折叠显示，第一个子视图必须是滚动视图（UICollectionView、UITableView、UIScrollView等）。
所以，我想......要从“发现”到“朋友”，让我们将z顺序交换为：

view.sendSubviewToBack(friendsCollectionView)

不幸的是，这并不起作用。显然，导航栏的功能是在添加视图时确定的。因此，所做的一切就是从两个集合视图中删除该功能：（
对于解决此问题的一种方法，我们可以：

• 使用.removeFromSuperview()删除两个集合视图
• 使用以下内容重新添加：
• view.insertSubview(friendsCollectionView, at: 0)
• view.insertSubview(discoveryCollectionView, at: 1)
• 重置其约束
• 设置其位置的动画

这里有一个简单的例子...
首先，简单的单标签单元格：

class SimpleCell: UICollectionViewCell {
    let label: UILabel = {
        let v = UILabel()
        v.textAlignment = .center
        v.translatesAutoresizingMaskIntoConstraints = false
        return v
    }()
    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }
    func commonInit() -> Void {
        contentView.addSubview(label)
        let g = contentView.layoutMarginsGuide
        NSLayoutConstraint.activate([
            label.topAnchor.constraint(equalTo: g.topAnchor),
            label.leadingAnchor.constraint(equalTo: g.leadingAnchor),
            label.trailingAnchor.constraint(equalTo: g.trailingAnchor),
            label.bottomAnchor.constraint(equalTo: g.bottomAnchor),
        ])
        label.layer.cornerRadius = 8
        label.layer.masksToBounds = true
    }
}

以及示例视图控制器：

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate {
    
    var discoveryCollectionView: UICollectionView!
    var friendsCollectionView: UICollectionView!
    
    var discoveryViewLeadingAnchor: NSLayoutConstraint!

    let segCtrl = UISegmentedControl(items: ["Discovery", "Friends"])

    @objc func segChanged(_ sender: Any?) {

        guard let segCtrl = sender as? UISegmentedControl else { return }
        
        // save the current "position" of discoveryCollectionView
        let curConstant: CGFloat = discoveryViewLeadingAnchor.constant
        
        discoveryCollectionView.removeFromSuperview()
        friendsCollectionView.removeFromSuperview()
        
        if segCtrl.selectedSegmentIndex == 1 {
            view.insertSubview(friendsCollectionView, at: 0)
            view.insertSubview(discoveryCollectionView, at: 1)
        } else {
            view.insertSubview(discoveryCollectionView, at: 0)
            view.insertSubview(friendsCollectionView, at: 1)
        }

        // we removed and re-added the collection views,
        // so we need to reset the constraints
        setCommonConstraints(discoveryLeading: curConstant)
        
        DispatchQueue.main.async {
            self.discoveryViewLeadingAnchor.constant = curConstant == 0.0 ? -self.view.frame.width : 0.0
            UIView.animate(withDuration: 0.3, animations: {
                self.view.layoutIfNeeded()
            })
        }
        
    }
    
    func setCommonConstraints(discoveryLeading c: CGFloat) {
        
        let g = view.safeAreaLayoutGuide

        // these constraints must be set each time the colletion views
        //  are added as subviews
        
        discoveryViewLeadingAnchor = discoveryCollectionView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: c)

        NSLayoutConstraint.activate([
            
            discoveryCollectionView.topAnchor.constraint(equalTo: g.topAnchor, constant: 0.0),
            discoveryCollectionView.bottomAnchor.constraint(equalTo: segCtrl.topAnchor, constant: -8.0),
            
            friendsCollectionView.topAnchor.constraint(equalTo: discoveryCollectionView.topAnchor, constant: 0.0),
            friendsCollectionView.bottomAnchor.constraint(equalTo: discoveryCollectionView.bottomAnchor, constant: 0.0),
            
            discoveryCollectionView.widthAnchor.constraint(equalTo: g.widthAnchor),
            friendsCollectionView.widthAnchor.constraint(equalTo: g.widthAnchor),
            
            friendsCollectionView.leadingAnchor.constraint(equalTo: discoveryCollectionView.trailingAnchor),

            discoveryViewLeadingAnchor,
            
        ])

    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.title = "Swap Testing"
        
        navigationController?.navigationItem.largeTitleDisplayMode = .automatic
        navigationController?.navigationBar.prefersLargeTitles = true
        
        let discoveryFL = UICollectionViewFlowLayout()
        discoveryFL.scrollDirection = .vertical
        discoveryFL.minimumLineSpacing = 12.0
        discoveryFL.minimumInteritemSpacing = 12.0
        
        let friendsFL = UICollectionViewFlowLayout()
        friendsFL.scrollDirection = .vertical
        friendsFL.minimumLineSpacing = 12.0
        friendsFL.minimumInteritemSpacing = 12.0

        // let's make the item height's different, just to confirm the functionality
        discoveryFL.itemSize = CGSize(width: 100.0, height: 60.0)
        friendsFL.itemSize = CGSize(width: 100.0, height: 100.0)

        discoveryCollectionView = UICollectionView(frame: .zero, collectionViewLayout: discoveryFL)
        friendsCollectionView = UICollectionView(frame: .zero, collectionViewLayout: friendsFL)
        
        discoveryCollectionView.translatesAutoresizingMaskIntoConstraints = false
        discoveryCollectionView.backgroundColor = .systemRed
        
        friendsCollectionView.translatesAutoresizingMaskIntoConstraints = false
        friendsCollectionView.backgroundColor = .systemBlue

        segCtrl.translatesAutoresizingMaskIntoConstraints = false

        view.addSubview(discoveryCollectionView)
        view.addSubview(friendsCollectionView)
        
        view.addSubview(segCtrl)
        
        let g = view.safeAreaLayoutGuide
        
        NSLayoutConstraint.activate([
            
            // let's put the segmented control at the bottom
            segCtrl.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -8.0),
            segCtrl.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            segCtrl.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            
        ])

        setCommonConstraints(discoveryLeading: 0.0)
        
        discoveryCollectionView.register(SimpleCell.self, forCellWithReuseIdentifier: "c")
        friendsCollectionView.register(SimpleCell.self, forCellWithReuseIdentifier: "c")

        discoveryCollectionView.dataSource = self
        discoveryCollectionView.delegate = self
        
        friendsCollectionView.dataSource = self
        friendsCollectionView.delegate = self
        
        segCtrl.selectedSegmentIndex = 0
        segCtrl.addTarget(self, action: #selector(segChanged(_:)), for: .valueChanged)
        
    }
    
    // so we can set the cell item width to the view width
    var cvWidth: CGFloat = 0
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

        // only want to call this when the frame changes
        if cvWidth != discoveryCollectionView.frame.width {
            cvWidth = discoveryCollectionView.frame.width
            if let fl = discoveryCollectionView.collectionViewLayout as? UICollectionViewFlowLayout {
                fl.itemSize.width = discoveryCollectionView.frame.width
            }
            if let fl = friendsCollectionView.collectionViewLayout as? UICollectionViewFlowLayout {
                fl.itemSize.width = friendsCollectionView.frame.width
            }
        }
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        // let's use different number of items, just to make sure everything is working right
        return collectionView == discoveryCollectionView ? 20 : 30
    }
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "c", for: indexPath) as! SimpleCell
        if collectionView == discoveryCollectionView {
            cell.label.text = "Discovery: \(indexPath.item)"
            cell.label.backgroundColor = .yellow
        } else {
            cell.label.text = "Friends: \(indexPath.item)"
            cell.label.backgroundColor = .cyan
        }
        return cell

    }
    
}